<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    
    <style>
        html, body {
            height: 100%;
            padding: 0px;
            margin: 0px;
        }
        body * {
            padding: 20px;
            box-sizing: border-box;
        }
        .head {
            height: 60px;
            background-color: aqua;
            position: absolute;
            width: 100%;
        }
        .atc {
            background-color: burlywood;
            overflow: auto;
            width: 100%;
            position: absolute;
            bottom: 100px;
            top: 60px;
            overflow: auto;
        }
        .left-nav {
            background-color: chocolate;
            float: left;
            width: 200px;
            height: 100%;
            overflow: auto;
            margin-right: 20px;
        }
        .sec {
            background-color: darkseagreen;
            overflow: auto;
            /* float: left; */
            /* position: absolute;
            left: 240px;
            right: 140px; */
            height: 100%;
            overflow: auto;
        }
        .right-sec {
            background-color: springgreen;
            float: right;
            width: 100px;
            height: 100%;
            overflow: auto;
        }
        .fot {
            background-color: black;
            color: white;
            height: 100px;
            position: absolute;
            width: 100px;
            bottom: 0;
            width: 100%;
        }
        .content-head {
            position: sticky;
            top: 0px;
        }
    </style>
</head>
<body>
    <header class="head">页面头部</header>
    <article class="atc">
        <nav class="left-nav">中间左侧导航</nav>
        <section class="sec">
            <h1>非固定头部</h1>
            <h2 class="content-head">滚动到顶部时固定到顶部</h2>
            <div style="height: 2000px;"></div>
        </section>
        <!-- <section class="right-sec">中间右侧导航</section> -->
    </article>
    <footer class="fot">页面底部</footer>
</body>
</html>